A radio tag serving as a radio transponder is a device made to transmit an electric wave having a modulated unique identification information, information stored in a storage unit or the like in response to a reception of an electric wave in a specified frequency band (for example, 13.56 MHz or 2.45 GHz). Radio tags are classified into an active type and a passive type. An active type radio tag can prepare power for itself to simplify a configuration of a reader/writer side apparatus serving as a radio interrogator. The latter cannot prepare power for itself, and conducts operations such as transmission of information by receiving energy from the external. The passive type is preferable from the viewpoint of reduction of cost of a radio tag and particularly has a bright future. When a radio signal in the above-mentioned specified frequency band is transmitted from the reader/writer, the passive type radio tag transmits an electric wave as a response by receiving the supply of power through the electromagnetic coupling by this radio signal.
Thus, the radio tag recognition on the reader/writer side, the readout of information stored in the radio tag and the writing of data in the radio tag can be made through the transmission/reception of data between the reader/writer and the radio tag. Incidentally, the radio tag is equally referred to as an IC (Integrated Circuit) tag, IC card, radio IC tag, electronic tag, transponder, RF (Radio Frequency) tag or RFID (Radio Frequency Identification).
Such a radio tag has already been applied to various fields including manufacturing industry, commercial distribution, transportation and medical treatment. For example, it is attached to various types of parts at a manufacturing stage moving on a line in a product manufacturing process in a factory so as to store a process history and others of each of the parts flowing on the line, thereby achieving efficient product manufacture management.
For the employment of a radio tag as mentioned above, depending upon utilization mode, there is a case in which, particularly, a plurality of readers/writers are disposed to be brought relatively close in distance to each other. For example, with respect to a reader/writer for reading a radio tag attached to a part moving on a manufacturing line as mentioned above, in the case of employing a factory layout in which they are brought close to each other even in different manufacturing processes, it is considered that the readers/writers provided in the respective processes are disposed at positions brought relatively close to each other.
In a case in which a plurality of readers/writers are disposed to be brought relatively close in distance to each other as mentioned above, it is considered that the electric waves (radio transmission signals) transmitted from the respective readers/writers interfere with each other and, in each reader/writer, due to the interference with a radio transmission signal transmitted from the other readers/writers, this works against the communication between radio tags and shortens the communicable distance.
For avoiding such troubles, considered is a first technique in which, for example, as shown in FIG. 13, the respective readers/writers (R/W) 201 to 204 use different frequency channels (f1 to f4) and transmit radio transmission signals to radio tags 211 to 214 which are objects of communication. That is, when a reception unit of each of the readers/writers 201 to 204 is configured to be capable of selectively receiving only a frequency channel to be used for a radio transmission signal, it is possible to reduce the influence of a transmission signal from a reader/writer adjacent thereto or a signal from a radio tag making a communication with the adjacent reader/writer.
In addition, for avoiding the above-mentioned troubles, there is a second technique in which, for example, as shown in FIG. 14, a plurality of readers/writers R/W#1 to R/W#4 successively make communications with radio tags (Tags) which are communication partners, respectively, in a time division fashion. While one reader/writer (for example, R/W#1) makes communications [transmits a command (Command) to a radio tag and receives a radio response signal (Response) from a radio tag], the other readers/writers (R/W#2 to R/W#4) do not make communications, which enables the readers/writers and the tags to avoid the interference with the other readers/writers.
In this connection as the well-known techniques related to the invention of the present application, there has been known the techniques disclosed in the following patent document 1 (Japanese Patent Laid-Open No. 2003-283367) and patent document 2 (Japanese Patent Laid-Open No. 2003-150916).
The patent document 1 discloses the technique in which a plurality of interrogators operating in synchronism with each other are provided so that the communicable areas of antennas overlap with each other, thereby enlarging the communicable areas of the interrogators without incurring interference, omission of detection and decrease in processing speed.
Moreover, the patent document 2 discloses the technique in which, for preventing the interference between radio tags•readers/writers, a function equivalent to a radio tag is added to the radio tags•readers/writers so that the radio tags•readers/writers themselves can behave as a radio tag with respect to a request from the other radio tags•readers/writers.
From the viewpoint of, in each reader/writer, avoiding the above-mentioned interference by a radio transmission signal transmitted from the other readers/writers, the first and second techniques create the following problems.
That is, in the case of the above-mentioned first technique, a radio tag side reception unit is usually designed so as to be capable of receiving signals in a plurality of channels or in different frequency bands, and when viewed from the radio tag side, a signal from the adjacent reader/writer directly causes an interference. That is, in the case of the simultaneous reception of radio signals from a plurality of readers/writers, an interference occurs with the transmission of a normal response to each reader/writer.
In addition, in the case of the above-mentioned second technique, this reader/writer cannot make a communication while the other reader/writer is in communication, which creates a problem of the reduction of throughput of the entire reader/writer. In this case, as the number of readers/writers disposed in a given area where radio transmission signals can interfere with each other increases, the throughput decreases in inverse proportion thereto.
The above-mentioned technique disclosed in the patent document 1 relates to a technique in which, by using a plurality of interrogators synchronized and antennas provided to make communication areas overlap with each other, a communicable area is enlarged to eliminate the detection omission with respect to a transponder moving on a conveyer. That is, this technique differs from a technique for avoiding the interference between radio transmission signals when a plurality of radio interrogators different from each other are simultaneously placed into operation.
Moreover, also in the technique disclosed in the patent document 2, since this reader/writer cannot make a communication while the other reader/writer is in communication, there is a problem in that the throughput of the entire reader/writer system decreases.
The present invention has been developed in consideration of these problems, and it is an object of the invention to provide a radio interrogator system and radio communication method, capable of, even in a case in which a plurality of radio interrogators are disposed at positions brought relatively close to each other and are simultaneously placed into operation, reducing the interference with a radio transponder and avoiding the decrease in throughput of the entire radio interrogator system.
[Patent Document 1] Japanese Patent Laid-Open No. 2003-283367
[Patent Document 2] Japanese Patent Laid-Open No. 2003-150916